The present invention relates to patch antennas, and more particularly to antennas using aperture coupling with symmetric conductive elements to generate circular polarization.
Typical aperture coupled patch antenna technology has most often been used in the defense and aerospace industries. However, aperture coupled patch antennas have recently been applied in low cost commercial applications such as global positioning satellites, paging, cellular communication, personal communication systems, global systems for mobile communication, wireless local area networks, cellular video broadcasting, direct broadcast satellites, automatic toll collection, collision avoidance radar, and wide area computer networks.
Aperture coupled patch antennas are generally designed to broaden the bandwidth of the operational input impedance to support the broader band services of cellular 800/900 MHz and personal communication systems (xe2x80x9cPCSxe2x80x9d) 1800/1900 MHz bands. These services incorporate the use of linearly polarized patch antenna arrays at the base stations and, in some configurations, in mobile or vehicular applications.
An exemplary aperture coupled microwave antenna is shown in U.S. Pat. No. 5,241,321 for xe2x80x9cDual Frequency Circularly Polarized Microwave Antennaxe2x80x9d to Tsao issued Aug. 31, 1993. Tsao discloses an antenna capable of generating circularly polarized signals. The antenna requires a dual feed approach to augment operation at two separate frequencies to achieve a xe2x80x9cdual frequencyxe2x80x9d mode antenna. The geometry places the feeds orthogonal to each other and each electromagnetically couples the aperture through the crossed slots. The crossed slots are essentially isolated electrically from each other so as not to interfere with one another. The antenna thus is an aperture fed patch via electromagnetic coupling from the feed circuits/aperture design. However, the square patch element requires the incorporation of tuning stubs for adjusting for optimal circularity of the polarization at each desired frequency. The conductive tuning stubs attached to the sides of the patch are operable to induce a 90 degree phase separation between dual linearly polarized signals to convert them into a circularly polarized signal. The stubs are either inductive or capacitive. Specifically, to achieve circular polarization, the antenna requires that the tuning stubs be directly attached to the patch element to convert two linearly polarized frequencies to a circular polarization. The tuning stubs thus require complex implementation and adjustment to accomplish circular polarization. The antenna also requires multiple dielectric layers, complicated feeding networks, and multiple ground layers to achieve certain characteristics.
Similarly, other antennas are structured and designed to achieve broad band coupling and circular polarization. For example, the antenna disclosed in U.S. Pat. No. 6,396,442 to Kawahata et al xe2x80x9cCircularly Polarized Antenna Device and Radio Communication Apparatus Using the Samexe2x80x9d issued May 28, 2002 discloses a circularly polarized antenna for a radio communication apparatus. The antenna includes a dielectric base, an electrode, feeder electrodes, and a feeder circuit board. Specifically, the antenna requires a complex feeding network, and four feeder electrodes in one embodiment, to achieve circular polarity. The complex feeding network requires complex implementation. The feeder electrodes further increase the difficulties in implementing such an antenna.
Still another antenna is disclosed in U.S. Pat. No. 6,166,692 to Nalbandian et al for xe2x80x9cPlanar Single Feed Circularly Polarized Microstrip Antenna with Enhanced Bandwidthxe2x80x9d issued Dec. 26, 2000. Nalbandian et al teaches a planar single feed circularly polarized microstrip antenna, which requires a multiple layer arrangement. In one embodiment, the antenna is formed by two layered cavities with two rectangular conductive patches. The antenna, similarly to the previously disclosed antennas, uses multiple layers and complicated feed networks to achieve circular polarization. While attempting to provide the desired low profile configuration and wide bandwidth, the antenna still require complicated structure and multiple layers thereby increasing the implementation difficulties.
As described, most of the aperture coupling work involves broad banding or dual banding the antennas to achieve specific performance goals for linear polarized patch configurations. Complex arrangements of coupling apertures and quadrature feed networks (polarizers) are often incorporated to generate orthogonal phasing to accomplish circular polarization. Furthermore, degradation occurs in the axial ratio or the radiation pattern when aperture coupling through a slot is used, and the corresponding gain also suffers when polarizers or other hybrid combining feed networks are utilized, which also leads to unnecessary feed loss.
Some of these antennas also incorporate offset fed square or circular patch elements, xe2x80x9calmost squarexe2x80x9d patches, slotted patches, crossed slot apertures, orthogonal coupling slots fed with quadrature feed, crossed slot within multiple layers and offset fed mitered patches. A substantial drawback associated with these designs is that they require either careful alignment or placement of the feed probe or the feed networks for proper coupling and circular polarization. Additionally, such designs are further limited in impedance or axial ratio bandwidth. While stacked patches or multiple layers are shown to achieve broad bandwidth, they fail to maintain a broad banded (i.e.  greater than 5%) axial ratio.
Accordingly, there is a need for an improved method and apparatus of transmitting and receiving broadcast signals with an antenna. Further, it would be beneficial to increase signal bandwidth percentage, to broaden signal bandwidth, to improve an axial ratio and a phase separation, and to optimize polarization of an antenna.
Consequently, the present invention provides a system of transmitting and receiving signals. In one embodiment, the invention provides an antenna that includes a substrate that has a first surface and an opposing second surface, and a first conductive element that is positioned at the first surface of the substrate. The first conductive element defines an aperture therein at the first surface of the substrate. The antenna also includes a conductive strip positioned at the opposing second surface of the substrate. The conductive strip is electrically isolated from the aperture by the substrate therebetween, and provides a transmission line that generates electromagnetic coupling with the aperture. Further, the antenna has a symmetric conductive element in the form of a planar polygon that is positioned relative to the aperture for broadband coupling of electromagnetic radiation. In addition, the opposing corners that are formed on the symmetric conductive element are configured to induce quadrature phasing.
In another embodiment, the present invention provides a method of radiating circularly polarized signals. The method includes providing a substrate that has a first surface and an opposing second surface, and positioning a first conductive element at the first surface of the substrate, wherein the conductive element defines an aperture. The method also includes positioning a conductive strip at the opposing second surface of the substrate, wherein the conductive strip is electrically isolated from the aperture by the substrate therebetween, and provides a transmission line that generates electromagnetic coupling with the aperture. Furthermore, the method includes positioning a symmetric conductive element relative to the aperture for broadband electromagnetic coupling and radiation. The symmetric conductive element is in the form of a planar polygon. The method also includes forming opposing corners on the symmetric conductive element wherein the opposing corners are configured to induce quadrature phasing, and feeding the conductive strip with a signal.
Briefly summarized, the invention provides a patch antenna structure including an aperture, a conductive strip and a symmetric conductive element to achieve circular polarization. The symmetric conductive element is spaced relative to the conductive strip, and the symmetric conductive element and the conductive strip are electromagnetically coupled through the aperture. The antenna also includes a first conductive element that defines the aperture therein at the first surface of the substrate. The conductive strip is positioned at an opposing second surface of the substrate. The conductive strip is electrically isolated from the aperture by the substrate therebetween, and, provides a transmission line that generates electromagnetic coupling with the aperture. Further, the symmetric conductive element is in the form of a planar polygon, and is positioned relative to the aperture and the conductive strip for broadband coupling of electromagnetic radiation. The antenna thus achieves optimal performance for gain, axial ratio and input impedance over relatively large bandwidth.
Other features and advantages of the invention will become apparent by consideration of the detailed description and accompanying drawings.